Lithographic systems are used to project patterns onto a substrate resting on a stage. An example of such a system can be seen in Resor et al. U.S. Pat. No. 4,769,680 (U.S. Pat. No. Re. 33,836). The substrate carries a photosensitive coating which is developed and processed after exposure. The same substrate may go through this cycle several times until the desired number of patterned layers have been built up. Since many layers may be used, it is necessary to be able to position and align the substrate accurately before each exposure.
The primary attributes of such an alignment system are accuracy and precision. Typically, these attributes are degraded by several error sources:
Excessive aberrations in the alignment optics (especially asymmetric aberrations); PA1 Focus sensitivity; focus walk; focus dependent aberrations; PA1 Susceptibility to variations in substrate optical properties, such as reflectivity, resist thickness, and color; PA1 Poor quality of alignment mark; PA1 Sensitivity to stray light and flare; PA1 Improper calibration; PA1 Instability of alignment mechanics (long-term drift); PA1 Noise.
Though the present invention senses light diffracted from an image, as does Wilczynski U.S. Pat. No. 4,232,969, Wilczynski projects its image through a hole in a mirror and uses the mirror to reflect the diffracted light, rather than detecting it along the optical axis. The mirror slows down the lithographic process since it has to be moved out of the way after each alignment step.